Atrial fibrillation is an irregular heart rhythm that adversely affects approximately 2.5 million people in the United States. It is believed that at least one-third of all atrial fibrillation originates near the ostium of the pulmonary veins. Anatomically, two pairs of pulmonary veins are connected to the left atrium of the heart with each pair delivering blood to the heart from one of the patient's lungs.
It is further believed that the optimal technique to treat atrial fibrillation is to create circumferential lesions around the ostia where a pulmonary vein connects with the left atrium. More specifically, the goal is to ablate tissue to form a conduction block to thereby prohibit the transmission of irregular electrical signals that can cause an arrhythmia. To be effective, the conduction block must completely block irregular signals and this often requires the ablation of a relatively deep, uniform lesion.
Heretofore, due to the relatively large diameters of these ostia, cryoablation procedures have typically been accomplished using multiple, successive contacts between a cryo-element and the tissue around the periphery of an ostium. More specifically, these procedures have required the cryo-element to be successively moved around the ostia to create a patchwork array of ablations. This often results in a non-uniform circumferential ablation that fails to form an adequate conduction block. Moreover, these multiple contact procedures tend to be complicated, time consuming, difficult to perform, and generally unreliable.
To simplify and quicken the procedure and produce a more uniform lesion, it is desirable to make circumferentially shaped lesions, either in a single contact or with as few contacts as possible. This, in turn, implies the use of a hoop shaped or partially hoop shaped contacting element. One approach to establishing a hoop shaped lesion is to use a so-called “passive” contacting element. In greater detail, this generally involves pre-forming a contacting element in a curved or hoop shape. The passive contacting element is then deformed into a straight configuration and passed through a guiding tube to reach the treatment site. At the site, the contacting element is emitted from the distal end of the guiding tube where it relaxes into its pre-formed shape.
Unfortunately, passive contact elements have several drawbacks. For one, it can be somewhat difficult to guide the deformed element through the tortuous vasculature, due in part to the resistance presented by the deformed contact element in the guide tube. In addition, the size (i.e. radius) of the hoop generated using a passive contact element is typically not adjustable in situ. This later drawback can be significant, for example, when more than one lesion is needed and different hoop sizes are required for each lesion.
In light of the above, it is an object of the present invention to provide systems and methods suitable for the purposes of cryoablating substantially circumferential ablations of internal tissue, while minimizing the number of tissue-cryo-element contacts. It is another object of the present invention to provide systems and methods for actively forming a hoop shaped contact element in a body conduit. It is yet another object of the present invention to provide systems and methods for cryoablating internal target tissue that can be performed quickly and are relatively reliable.